XP from a CMM perspective by Mark Paulk, who is one of the leaders of the CMM community. The original resides here (the website of the Software Engineering Institute - SEI)

The Agile Manifesto by Martin Fowler and Jim Highsmith. The Agile methodologies manifesto explained. The article is from the SD Magazine (Software Development Magazine). The original is here(requires registration which is free).

Thinking in Patterns with Java is an wonderful online book by Bruce Eckel (the author of "Thinking in Java", "Thinking in C++", "Thinking in C#", "Thinking in Python" - also known as the "Thinking books"). It is available for download and is in HTML format (one big file). Quite useful to those who want to get an idea about design patterns, or want to see sample code in Java about design patterns. The DP bible by GoF (Gang of Four - E. Gamma et al.) gives only examples in C++ and SmallTalk.